Zoom H2: Spontaneous Speed Change

We were recording 2 isolated channels, Line-in, from a Sound Devices 302 mixer. File format: WAV 44.1 / 16. The first session (about 3 hrs) went well and sounded great after transfer to computer. Four minutes into the start of the next session, without any indication in the monitoring headsets or restart of the recorder, the record speed was cut by 50% (we had to adjust speed to 200% in Final Cut to correct the recording). How can this happen? When the file is played back, you can hear a slight "click" at the moment the speed changes. We made a test recording later and it was normal.
Thanks,
Bob

If it hasn't happened before or since chalk it up to a one time anomolie. For the future keep your eye on the display screen and note any change in the time remaining window as an indication of a slower speed.